Enzymes which are capable of N-acetylating serotonin or tryptamine were partially isolated from rat pineal gland and rat liver. Both enzymes displayed ping-pong kinetics suggestive of an acetylated enzyme intermediate. The molecular weight of the liver enzyme was 26,000 daltons while the pineal gland enzyme was found to be 39,000 daltons. The reagent beta-mercaptoethylamine appeared to dissociate both the liver and pineal enzymes to 10,000-12,000 dalton subunits. It is suggested that the active liver enzyme is a dimer of 12,000 dalton subunits while the active pineal enzyme is a tetramer of 10,000 dalton subunits. BIBLIOGRAPHIC REFERENCE: Morrissey, J.J., Edwards, S.B. and Lovenberg, W.: Comparison of rat pineal gland and rat liver N-acetyltransferase. Biochem. Biophysical. Research Comm. In press (1977).